Polybutylene terephthalate (PBT) resins are a well known class of thermoplastic engineering resins and are used extensively to form a variety of molded articles due to its favorable mechanical, electrical and chemical properties, as well as its relative ease of processability. Although PBT resins may be use alone to form molded articles, it is typically blended with one or more additive components so as to tailor the properties of the resulting blended PBT composition to a particular end-use application. For example, fibrous reinforcement agents are typically blended with PBT base resins to form reinforced molded PBT articles. These "filled" PBT resin molded articles (i.e., containing reinforcement agents) are thereby intended to be employed in end-use applications which require greater mechanical strength and rigidity characteristics than can be attained by the use of "unfilled" PBT base resins alone.
PBT compositions which are filled with such fibrous reinforcing agents typically exhibit a relatively high degree of mechanical strength and rigidity as compared to unfilled PBT resins which is apparently due to the formation of anisotropic regions during molding. However, the inclusion of fibrous reinforcing agents is problematic in that increased warpage of molded products--especially relatively thin planar products or box-shaped products having relatively thin wall sections--is to be expected.
Several techniques have been proposed to minimize the warping propensity of molded articles made from PBT resin compositions which are filled with reinforcing agents. For example, it has been suggested that PBT compositions containing spherical reinforcing agents (e.g., in the form of glass beads) or plate-like reinforcing agents (e.g., in the form of mica) may be used to reduce the expected warpage of molded articles. These suggested proposals typically do result in some molded articles having a considerably lesser amount of warpage. However, problems still exist when PBT compositions are employed to form molded articles having relatively thin walls.
For example, in the case of relatively thin planar articles (such as floppy discs) and box-shaped articles having relatively thin walls (such as thin-walled electrical connectors, microswitches, miniature coil bobbins, computer keyboard frames and the like), mold defects typically occur due to inadequate melt-flow properties of PBT compositions filled with reinforcing agents. This inadequate melt-flow property of reinforcement-filled PBT resin compositions thus results in non-uniform flow of the molten resin within the mold cavity with the usual result that the mold cavity is inadequately filled. As a consequence, therefore, thin-walled articles molded from reinforcement-filled PBT resin compositions may have nonuniform regions which are more susceptible to warpage.
Thus, a need exists in this art for PBT resin compositions which are filled with a reinforcing agent to have improved melt-flow characteristics which would be better suited for use in forming thin-walled molded articles. It is towards fulfilling this need that the present invention is directed.
According to the present invention, filled PBT resin compositions are provided which exhibit superior melt-flow properties so as to be useful for forming thin-walled molded articles. The resulting thin-walled molded articles have desirable mechanical properties and an extremely low degree of warpage. The present invention thus offers thin-walled molded articles made of PBT resin which have a low degree of warpage.
The present invention more specifically relates to PBT resin compositions and thin-walled molded articles formed thereof which include a PBT base resin having a specifically defined viscosity, plate-like glass flakes having a specifically defined flake dimension, and a metal salt of a fatty acid.
Further aspects and advantages of the present invention will become more apparent after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.